(1) Field of the Invention
The invention relates to gas turbine engine components in general, and specifically to a seal for preventing leakage of high pressure air or other fluids between segmented components used in such engines.
(2) Description of the Related Art
As illustrated in FIG. 1, a gas turbine engine 10 comprises one or more forward compressor sections 12, a central combustor section 14 and one or more rearward turbine sections 16. The engine 10 operates by compressing ambient air 18 with the compressors 12, adding fuel upstream of the combustor 14 and burning a fuel-air mixture 20 in the combustor 14. High temperature combustion gases 22 are directed axially rearward from the combustor 14, through an annular duct 24 disposed in the turbines 16. The combustion gases 22 interact with one or more turbine rotors 26, outer portions of which are disposed in the duct 24. The turbine rotors 26 are coupled to compressor rotors 28 via concentric shafts 30 rotating about a central longitudinal axis 32 of the engine 10. Gas turbine engines are known to power aircraft, ships and electrical generators.
Extending into the annular gas duct 24 are alternating circumferential stages of rotating blades 34 and stationary vanes 36. The stationary vanes 36 extend radially inwardly from a casing structure 38 surrounding the turbines 16. To prevent oxidation of the vanes 36 and other stationary components due to the hot combustion gases 22, low temperature compressor air 40 is directed radially inboard and outboard of the duct 24 to the components. The compressor air 40 is maintained at a higher pressure than the combustion gas 22 pressure, to ensure a continuous supply of compressor air 40 reaches the components.
Because stationary components such as vanes, shrouds, supports and the like are subject to extreme temperature gradients, they can develop cracks due to thermal mechanical fatigue (TMF). To reduce the occurrence of TMF induced cracking, these components are typically installed in semi annular segments distributed circumferentially about the engine's longitudinal axis. The segmented components are uncoupled from one another, thus allowing them to expand and contract independently. In addition to their improved TMF resistance, segmented components are also less expensive to repair and/or replace after extended use.
Despite the aforementioned benefits, axial and radial gaps must be included between adjacent components to allow for thermal expansion. These gaps require sealing to ensure an adequate pressure differential exists between the compressor air and the combustion gas. Maintaining a compressor air pressure that is greater than the combustion gas pressure ensures a continuous flow of compressor air and prevents backflow of the combustion gas. Excessive leakage of the compressor air may cause premature oxidation of the components and can increase the engine's fuel burn. With jet fuel accounting for up to sixty five percent of the operating expense of a commercial airliner, any reduction in fuel burn is beneficial.
Various seal configurations are known to restrict leakage of a pressurized fluid through a gap between two components. Feather seals are the type most commonly used between segmented components in gas turbine engines. Feather seals comprise a slot in the adjacent components that are open to the gap, and a bridging element disposed in the slots, spanning across the gap.
Flat bridging elements, such as those disclosed in U.S. Pat. No. 5,154,577 to Kellock, et al, are fit into the adjoining slots. They depend on the higher-pressure compressor air to seat the bridging elements against the slots to form the seal. Assembly damage, misaligned slots, slot surface finish and low compressor air pressure may negatively affect the performance of flat bridging elements.
Resilient bridging elements, such as those disclosed in U.S. Pat. No. 4,537,024 to Grosjean, are press fit into the adjoining slots. They rely on the contact pressure between the bridging element and the slot being greater than the compressor air pressure to form the seal. However, the single loop ends of the bridging elements offer limited contact pressure with the slot and are subject to compression about their minor axis.
Although misaligned slots, slot surface finish and low compressor air pressure have a less negative impact on resilient bridging elements, feather seal improvement is still needed.